Bi-level digit line architecture for high density DRAMS

ABSTRACT

There is a bi-level bit line architecture. Specifically, there is a DRAM memory cell and cell array that allows for six square feature area (6F 2 ) cell sizes and avoids the signal to noise problems. Uniquely, the digit lines are designed to lie on top of each other like a double decker overpass road. Additionally, this design allows each digit line to be routed on both conductor layers, for equal lengths of the array, to provide balanced impedance. Now noise will appear as a common mode noise on both lines, and not as differential mode noise that would degrade the sensing operation. Furthermore, digit to digit coupling is nearly eliminated because of the twist design.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.10/155,132, filed May 23, 2002, pending, which is a continuation ofapplication Ser. No. 09/941,201, filed Aug. 28, 2001, now U.S. Pat. No.6,456,518 B1, issued Sep. 24, 2002, which is a divisional of applicationSer. No. 09/533,353, filed Mar. 23, 2000, now U.S. Pat. No. 6,429,529B1, issued Aug. 6, 2002, which is a continuation of application Ser. No.09/211,662, filed Dec. 15, 1998, now U.S. Pat. No. 6,084,307, issuedJul. 4, 2000, which is a continuation of application Ser. No.08/950,471, filed Oct. 15, 1997, now U.S. Pat. No. 5,864,181, issuedJan. 26, 1999, which is a continuation of application Ser. No.08/442,264, filed May 15, 1995, abandoned, which is a continuation ofapplication Ser. No. 08/123,027, filed Sep. 15, 1993, abandoned.

BACKGROUND OF THE INVENTION

[0002] Field of the Invention: The present invention relates tointegrated circuits (ICs). Particularly, there is a RAM device wheredigit and digit bar, defined as a pair, are laid out vertically (in thez-axis) to each other, whereas the pairs of digit lines are laid out tobe parallel (in the x or y axis) to each other. Additionally, thevertically aligned digit line pairs allow usage of memory cells having asix square feature area (6F²) or less, where F is defined as the minimumrealizable photolithographic process dimension feature size.

[0003] Dynamic random access memory (DRAM) production in the early daysresulted in large chips. Manufacturing of these chips, at first, was notconcerned with shrinking every part down to its smallest size. At thistime the open memory array was the standard design: true digit lines onone side and complement digit (also known as digit bar or digit*) lineson the opposite side, with sense amps in the middle. However, once theDRAMS reached the 256K memory density, shrinking of all features becameimportant.

[0004] However, to push to even higher densities, like a one Megabitdensity, the open architecture proved to be inadequate because of thepoorer signal-to-noise problem. As a result, the folded bit linearchitecture was developed. Yet, to use this architecture, the originalmemory cell from the open architecture could not be used. Thus, newcells were designed. There resulted a memory cell with a minimum size ofeight square feature area (8F²). The folded architecture eliminated thesignal to noise problems. Thus, further shrinkage of the othercomponents on the DRAM resulted in an overall smaller DRAM package.

[0005] Problem

[0006] For some time now, there have been many ways developed to shrinkthe die size. However, a new shrinkage barrier has been reached asdesigns approach densities of 16 and 64 Meg chips. Every aspect of thedie now has to be designed with minimal size. Thus, it is now necessaryto shrink the previously acceptable eight square feature area (8F²)cells. Cell sizes of six square feature area (6F²) to four squarefeature area (4F²) are now needed. As a result, customers now needmemory cells of six square feature area (6F²) or smaller that will alsoavoid the previous signal to noise ratio problems.

[0007] Note, the above-described problem, as well as other problems, issolved through the subject invention and will become more apparent, toone skilled in the art, from the detailed description of the subjectinvention.

BRIEF SUMMARY OF THE INVENTION

[0008] One skilled in the art will appreciate the advantage of thebi-level bit line architecture. Specifically, there is a DRAM memorycell and cell array that allows for six square feature area (6F²) cellsizes and avoids the signal to noise problems. Uniquely, the digit linesare designed to lay on top of each other like a double decker overpassroad. Additionally, this design allows routing of digit lines on bothconductor layers, for equal lengths of the array, to provide balancedimpedance. Now noise will appear as a common mode noise on both lines,and not as differential mode noise that would degrade the sensingoperation. Furthermore, digit to digit coupling is nearly eliminatedbecause of the twist design.

[0009] To achieve the digit line switching, several modes of verticaltwisting were developed. For a given section of the array, the twistsare alternated between adjacent digit line pairs such that the overalltwist resembles the traditional folded digit line twist. This twistingof the lines ensures that the signal to noise ratio of the bi-leveldigit line architecture can be as good as or may be even better than thefolded digit line.

[0010] Other features and advantages of the present invention may becomemore clear from the following detailed description of the invention,taken in conjunction with the accompanying drawings and claims, or maybe learned by the practice of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0011]FIG. 1 is an illustration of one embodiment of the invention.

[0012]FIG. 2 is an illustration of one embodiment of the invention.

[0013]FIG. 3 is an illustration of one embodiment of the invention.

[0014]FIG. 4 is an illustration of one embodiment of the invention.

[0015]FIG. 5 is an oblique view of a portion of the memory array showingthe location of the twists, sense amps, row decoders, and isolationdevices.

[0016]FIG. 6 is a representation of the layout of the present invention.

[0017]FIG. 7 is a suggested layout for a portion of a DRAM memory arrayhaving twisted double-layer digit line pairs.

[0018]FIG. 8 is an alternative suggested layout for a portion of a DRAMmemory array having twisted double-layer digit line pairs.

[0019] It is noted that the drawings of the invention are not to scale.The drawings are merely schematic representations, and not intended toportray specific parameters of the invention. The drawings are intendedto depict only typical embodiments of the invention and are, therefore,not to be considered limiting of its scope. The invention will bedescribed with additional specificity and detail through the use of theaccompanying drawings, specification, and claims. Additionally, likenumbering in the drawings represents like elements within and betweendrawings.

DETAILED DESCRIPTION OF THE INVENTION

[0020] Incorporated Material

[0021] The following U.S. patents are herein incorporated by referencefor pertinent and supporting information:

[0022] U.S. Pat. No. 5,208,180, is a method of forming a capacitor.

[0023] U.S. Pat. No. 5,206,183, is a method of forming a bit line over acapacitor array of memory cells.

[0024] U.S. Pat. No. 5,138,412, is a dynamic RAM having an improvedlarge capacitance.

[0025] U.S. Pat. No. 4,742,018, is a process for producing memory cellshaving stacked capacitors.

[0026] U.S. Pat. No. 4,970,564, is a semiconductor memory device havingstacked capacitor cells.

[0027] U.S. Pat. No. 4,536,947, is a CMOS process for fabricatingintegrated circuits, particularly dynamic memory cells with storagecapacitors.

[0028] General Embodiment

[0029] One skilled in the DRAM semiconductor memory cell history and artwill easily understand the operation of this Bi-Level Digit line designusing an open architecture memory cell of six square feature area (6F²)or smaller feature size and switching of the digit line levels toeliminate the signal to noise ratio problems of the past.

[0030] This invention provides a new architecture for a dynamic randomaccess memory (DRAM). The memory is characterized as having a pluralityof digit line pairs, with each digit line pair consisting of both a truedigit line and a complement digit line. Both digit lines of each digitline pair are electrically insulated from one another by a dielectriclayer and vertically aligned along a major portion of their lengths. Atone or more positions along their lengths, their positions with respectto one another are reversed. In other words, if the true digit line isinitially on top during a first portion of the full length of the pair,the complement digit line is on the bottom and makes contact to aplurality of cells by means of digit line contacts. Using one of thetwisting techniques depicted in FIGS. 1 to 4, the complement digit lineis brought to the uppermost position while the true digit line isbrought to the lowermost position.

[0031] Further illustrated in FIG. 7 are isolation gates/lines 83 whichkeep the two adjacent memory cells from biasing each other. Suchisolation gates/lines 83 are grounded and are formed of polysiliconand/or other material, such as an insulator material. By having suchisolation gates/lines 83 grounded, the adjacent memory cells may be moreeffectively prevented from biasing each other during operation whilehaving higher potentials applied thereto.

[0032] Referring to drawing FIG. 8, an alternative embodiment of thedigit line switching, using vertical twisting, is illustrated. Asillustrated, with respect to digit line pair DP0 including upper digitline D* and lower digit line D, both metal digit lines, the right-handportion of upper digit line D* is connected by means of right standardcontact 94 to polysilicon area 90 and connected by means of leftstandard contact 94 from the polysilicon area 90 to the left-handportion of upper digit line D* while lower digit line D is insulatedfrom the polysilicon area 90 passing thereabove and thereover. Whenconsidering digit line pair DP1, upper digit line D* extends to cross orto overlie a portion of lower digit line D of digit line pair DPO,extends to bit contact 96, and extends over left standard contact 94,being insulated therefrom at the upper level of the digit line pair DP1of the array while the right-hand portion of lower digit line D of thedigit line pair DP1 extends to right standard contact 94, in turn,connected to N+ active area 92, in turn, being connected by leftstandard contact 94 to the left-hand portion of the lower digit line Dof the digit line pair DP1. In each instance, when considering the rightstandard contact 94, prior to such contact, both digit lines D* and Dare located vertically with respect to each other prior thereto in thearray and when considering the left standard contact 94, from thereonboth digit lines D* and D are located vertically with respect to eachother in the array. Furthermore, the pattern for the arrangement of thedigit lines is repeated with respect to digit line pairs DP2, DP3, DP4,and DP5 as described herein. In this manner, the noise is balancedthrough the use of vertical twists of the digit line pairs and the useof polysilicon areas and active N+ areas of the array. Additionallyillustrated and described herein are grounded gate isolation areas 83,word lines 82, and bit line contacts 81.

[0033]FIG. 1 illustrates one embodiment of the vertical three-leveldownward twist design to achieve equal bit line lengths on the top andbottom of the design. As illustrated, on the left side of the figure, D(digit) line 10 (also referred to as “plane 10”) is located directlyabove D* (D bar) line 12 (also referred to as “plane 12”). It is notedthat D line 10 drops down to a first plane 14, then to a third plane 16,and is routed around the D* line 12 and then elevated back up to thefirst plane 14. At the second level, D line 10 has achieved a twist inthe vertical direction or Z-axis. A similar vertical rotation occurs forD* line 12, except it drops down only one level to plane 18 and proceedsaround the third plane 16 location and then elevates to a same plane 12,and then to plane 22, where it will remain until the next twist isencountered.

[0034] It is noted that planes 10 and 22 are on the same level, as wellas planes 12 and 14, and planes 16 and 18, respectively. It also isnoted that all of the twisting is relatively in a z direction and thatat only two points does the twisting require additional X-Y planereal-estate, that being on levels 18 and 16.

[0035] Review of FIG. 2 shows almost an identical twist. However, thereare four levels in this twist. Level 4, or plane 19, is located belowlevel 3 and plane 16. Level 4 could be any material, like substrateimplant, polysilicon, metal 1, etc., the key factor being that planes 19and 16 don't create a transistor. A variation of this design is to haveplane 19 arranged like plane 18 in FIG. 1 to avoid a transistor if thematerial would create such.

[0036] Review of FIG. 3 illustrates a three-level twist-up architecture.As illustrated, the two digit lines are on the bottom planes 12, 14, 16and 18, whereas the twisting takes place on the upper planes 10 and 22.Again, all the planes are in a vertical orientation to one another.However, planes 10 and 22 do project out into the X-Y planes toaccomplish the twist.

[0037] Review of FIG. 4 illustrates a four-level downward twist. Digitline (D) 30 is moved down one level via planes 32, 34, and 36 whiledigit bar (D*) is twisted upward via planes 42 to 40. It is noted thatplane or line 42 is the only plane to extend in the X-Y plane, and, infact, it extends into the vertical plane of an adjoining pair of digitlines. To accommodate this extension, the bottom line 48 of D* is movedto a fourth lower level or plane 50, and then brought back up to line52, while digit line 46 has no need to be repositioned since it iselevated above the plane 42.

[0038] Review of FIG. 5 illustrates a DRAM and an oblique view of twosections of the array utilizing the bi-level twist architecture. It isnoted that, although there are two digit line pairs illustrated, theyare in fact vertically oriented, one lying on top of the other.Additionally, the X 68 marks illustrate where the twisting takes place.It is noted that each line in each pair will spend 50% of the lengthlocated on the bottom of the vertical architecture. For example, upperline 60 switches to lower line 66 and lower line 64 moves up to theupper line 62. Of course, the appropriate memory cells will be locatednear the correct bit line sections to receive the information stored inthe cells and feed that into the sense amps 70. An advantage with thisarchitecture is that the row decoders 72, attached to the row lines 73,can be positioned on one side of the array. Additionally, the isolationlines 74 are also symmetrical per array and thus can share a commongrounding node 76 located between the two arrays illustrated.

[0039] Attending to FIG. 6, there is an overview of a DRAM exhibitingeight memory cells 84 and the appropriate lines as illustrated. Inparticular, there is active area 80 running the length of bit lines 86(though one line is shown, both the D and D* lines are verticallyoriented). Word lines 82 will turn on the transistor to access thecells. Bit line contacts 81 will dump the cell charge onto the lower ofthe digit lines. Isolation gates/lines 83 keep the two adjacent memorycells from biasing each other.

[0040] Referring now to FIG. 7, a layout portion of a DRAM array havingdouble-layer twisted digit lines is depicted. Six digit line pairs (DP0,DP1, DP2, DP3, DP4 and DP5) are shown in this abbreviated layout. Itwill be noted that in the depicted portion of the array, only digit linepairs DP0, DP2 and DP4 undergo a twist. Digit line pairs DP1, DP3 andDP5 are untwisted in this portion of the array. The alternating twistpattern not only provides for efficient reduction of capacitive couplingbetween adjacent digit line pairs, but it also provides room for thetwisting operation. It will be noted that portions of first conductivestrip S1 and second conductive strip S2 are vertically aligned withportions of adjacent digit line pairs. This is possible because firstand second conductive strips S1 and S2 are not on a level with either ofthe adjacent double-layer digit lines. The memory cell layout to theright and left of the twist region 71 is similar to that depicted inFIG. 6. Vertical contact vias are represented by the squares marked withan “X”. The interconnect pattern is similar to that depicted in FIG. 1.In FIG. 1, Level 2, the digit lines located on planes 12 and 14 would beused to interconnect the corresponding pairs of adjacent contact vias.For example, for digit line pair DP2, the digit line located on plane 14would interconnect contact vias CV1 and CV2, while the digit linelocated on plane 12 would interconnect contact vias CV3 and CV4.

[0041] Remarks about the Invention

[0042] It is noted that the signal to noise ratios are kept acceptablylow. The vertical arrangement and the crossing digit lines allow forequal top and bottom orientation and access to the appropriate memorycells. Additionally, the adjoining pair of digit lines is also switchedappropriately to diminish signal to noise problems.

[0043] It is further noted that this array arrangement allows for thesmaller cell sizes, for example, cells possible from the older open bitline architecture or any new six square feature area (6F²) or smallercell size, thus allowing smaller arrays using six square feature area(6F²) to four square feature area (4F²) cell sizes.

[0044] A still further advantage is the overall arrangement of thecells, bit lines, word lines, and isolation lines. All devices and linesare laid out to be exactly straight. There is no routing around thecells to open the gates like with the eight square feature area (8F²)designs of the folded array structures.

[0045] Additionally, there is one sense amp (S-amp) located on one endof the digit and digit bar lines in an alternating pattern of the S-amp.

[0046] It is also noted that the twisting locations in the array are atquarter marks, either the first and third quarter, or at the halfwaymark in the array. This allows for different digit line pairarrangements to be located next to each other.

[0047] Variations in the Invention

[0048] There are several obvious variations to the broad invention whichthus come within the scope of the present invention. Uniquely, thisinvention may work with any positioning of the memory cells.Specifically, the cells may be located between, along side, on top, orunderneath the bit lines, thus accommodating for trench, stacked, orelevated designs. One skilled in the art would have little trouble usingthe vertical bi-level bit line arrangement with these other DRAMdesigns.

[0049] Additionally, any layering can be used for the bi-level digitlines. Specifically, the bottom layer could be an implant in thesubstrate, or poly on top of the substrate, or any of the metals overthe poly. It all depends on how high the chip design is stacked andwhere the memory cells are located.

[0050] Similarly, the twisting of the vertical digit lines can belocated anywhere in the array, like over {fraction (1/12)} of the line.The only requirement is that half of the length of each digit line islocated on top and half on the bottom of the vertical arrangement,although it is noted that any increase in the number of twists willincrease the size of the array.

[0051] While the invention has been taught with specific reference tothese embodiments, someone skilled in the art will recognize thatchanges can be made in form and detail without departing from the spiritand the scope of the invention. The described embodiments are to beconsidered in all respects only as illustrative and not restrictive. Thescope of the invention is, therefore, indicated by the appended claimsrather than by the foregoing description. All changes which come withinthe meaning and range of equivalency of the claims are to be embracedwithin their scope.

What is claimed is:
 1. A method for reducing differential noise in asemiconductor device having an integrated circuit memory having aplurality of memory cells, comprising: fabricating a first digit linehaving a width and a second digit line having a width, the first digitline and the second digit line extending in a first conductive level anda second conductive level above a plane extending along a portion of asurface of the integrated circuit memory device such that the firstdigit line and second digit line are substantially vertically alignedwithin the widths thereof and vertically stacked one from the other inthe plane; providing a vertical conductive twist to locate a portion ofeach of the first digit line and the second digit line in both the firstconductive level and the second conductive level; coupling an equalnumber of the plurality of memory cells of the integrated circuit memorydevice to portions of the first digit line and the second digit linelocated in a lower conductive level of the first conductive level andthe second conductive level; electrically balancing the first digit lineand the second digit line when vertically stacked one atop the other;and isolating a plurality of adjacent memory cells of the pluralityusing an isolation region.
 2. The method of claim 1, wherein saidisolating further comprises: isolating a plurality of adjacent memorycells of the plurality using an isolation region including isolationtransistors, each isolation transistor having a gate biased to apredetermined voltage.
 3. A method for differential noise reduction foran integrated circuit memory having a plurality of memory cells of asemiconductor device, comprising: fabricating a first digit line havinga width and a second digit line having a width, the first digit line andthe second digit line extending in a first conductive level and a secondconductive level above a plane extending along a portion of a surface ofthe integrated circuit memory device such that the first digit line andsecond digit line are substantially vertically aligned within the widthsthereof and vertically stacked one from the other in the plane;providing a vertical conductive twist to locate a portion of each of thefirst digit line and the second digit line in both the first conductivelevel and the second conductive level; coupling an equal number of theplurality of memory cells of the integrated circuit memory device toportions of the first digit line and the second digit line located in alower conductive level of the first conductive level and the secondconductive level; electrically balancing the first digit line and thesecond digit line when vertically stacked one atop the other; andisolating a plurality of pairs of adjacent memory cells of the pluralityusing an isolation region.
 4. The method of claim 3, wherein saidisolating further comprises: isolating a plurality of pairs of adjacentmemory cells of the plurality using an isolation region includingisolation transistors, each isolation transistor having a gate biased toa predetermined voltage.
 5. A noise reduction method for a memory devicehaving a plurality of memory cells of a semiconductor device,comprising: fabricating a first digit line having a width and a seconddigit line having a width, the first digit line and the second digitline extending in a first conductive level and a second conductive levelabove a plane extending along a portion of a surface of the memorydevice such that the first digit line and second digit line aresubstantially vertically aligned within the widths thereof andvertically stacked one from the other in the plane; providing a verticalconductive twist to locate a portion of each of the first digit line andthe second digit line in both the first conductive level and the secondconductive level; coupling an equal number of the plurality of memorycells of the memory device to portions of the first digit line and thesecond digit line located in a lower conductive level of the firstconductive level and the second conductive level; electrically balancingthe first digit line and the second digit line when vertically stackedone atop the other; and isolating a plurality of adjacent memory cellsof the plurality using an isolation region.
 6. The method of claim 5,wherein said isolating further comprises: isolating a plurality ofadjacent memory cells of the plurality using an isolation regionincluding isolation transistors, each isolation transistor having a gatebiased to a predetermined voltage.